MOLECULAR COMPOUNDS 147 



three equal minima in the direction of the axes (edges of 

 the cube) and four equal maxima at right angles to the 

 octahedral faces ; the figure which represents the hardness 

 in all directions round a point by radii proportional to it 

 has the symmetry of the cube form, i.e. three planes of 

 symmetry at right angles to one another, and six others 

 which pass through the lines of intersection and bisect the 

 angles between the first three planes. 



6. Molecular Compounds. 



A concluding remark may be appended here. The pos- 

 sibility of measuring molecular weights, due to the theory 

 of solutions, leads to the conclusion that the molecules are 

 usually of the smallest size that is in agreement with the 

 chemical data. In crystals, too, so far as the theory of 

 solid solutions (p. 70) has been applied, no higher molecular 

 complexes seem to occur. By the side of this the fact 

 appears somewhat surprising that in hydrates, e. g. Na 2 SO 4 

 . ioH 2 O, molecules are found possessing a high degree of 

 complexity. These seem, however, restricted to the solid 

 state, and certainly highly complex liquid molecules have 

 not been found to exist ; on the contrary, everything points 

 to such hydrates breaking down in solution. The inter- 

 laced spatial gratings of Sohncke's point systems allow 

 a conception of such hydrates, according to which the 

 different molecules would belong to different gratings. It 

 is important for this view that some such hydrates can 

 lose their water of crystallization without destruction of 

 the crystalline form, although their internal physical 

 properties are changed ; they take up water again on being 

 placed in their former conditions. Mallard 1 and Klein * 

 observed this phenomenon with certain aqueous silicates, 

 the zeoliths. Lately the same thing has been observed by 



1 Bull, de la Soc. mineral, de France, 5. 255. 



2 Ztitschr.f. Krystallographie, 9. 38. 



K 2 



